Turbines and other forms of commercial equipment frequently include rotating components inside or proximate to stationary components. For example, a typical gas turbine includes a compressor at the front, one or more combustors radially disposed about the middle, and a turbine at the rear. The compressor includes multiple stages of stationary vanes and rotating blades. Ambient air enters the compressor, and the stationary vanes and rotating blades progressively impart kinetic energy to the air to bring it to a highly energized state. The working fluid exits the compressor and flows to the combustors where it mixes with fuel and ignites to generate combustion gases having a high temperature and pressure. The combustion gases exit the combustors and flow through the turbine. A casing generally surrounds the turbine to contain the combustion gases as they flow through alternating stages of fixed blades or nozzles and rotating blades or buckets. The fixed blades or nozzles may be attached to the casing, and the rotating blades or buckets may be attached to a rotor. As the combustion gases flow through the nozzles, they are directed to the buckets, and thus the rotor, to create rotation and produce work.
The clearance between the casing and the rotating blades or buckets in the turbine is an important design consideration that balances efficiency and performance on the one hand with manufacturing and maintenance costs on the other hand. For example, reducing the clearance between the casing and the rotating buckets generally improves efficiency and performance of the turbine by reducing the amount of combustion gases that bypass the rotating buckets. However, reduced clearances may also result in additional manufacturing costs to achieve the reduced clearances and increased maintenance costs attributed to increased rubbing, friction, or impact between the rotating buckets and the casing. The increased maintenance costs may be a particular concern in turbines in which the rotating buckets rotate at speeds in excess of 1,000 revolutions per minute, have a relatively large mass, and include delicate aerodynamic surfaces. In addition, reduced clearances may result in excessive rubbing, friction, or impact between the rotating buckets and the casing during transient operations when the casing expands or contracts at a different rate than the rotating buckets during startup, shutdown or other variations in operation.
Conventional turbine casings generally include an outer turbine shell that holds the shrouds and nozzles. The outer turbine shell may surround one or more inner turbine shells. In some instances, each stage of rotating buckets has a separate inner turbine shell. The inner turbine shell is often split into two hemispherical shells joined or bolted together by flanges on a horizontal plane to facilitate maintenance and repair. During transient operations, temperature changes in the turbine produce axial and radial temperature gradients in the turbine casings. For example, during start up operations, the inner surfaces of the turbine shell heat up faster than the outer surfaces of the turbine shell, causing the inner material to expand faster than the outer material. As the inner material expands, the turbine shell bends to expand more horizontally than vertically, creating a slight horizontal out-of-roundness in the turbine shell. Conversely, during shutdown operations, the inner turbine shell cools down faster than the outer turbine shell, and the bolted flanges allow the inner turbine shell to contract more horizontally than vertically, again creating a slight vertical out-of-roundness in the inner turbine shell. Therefore, both startup and shutdown operations produce out-of-round conditions in the inner turbine shell that change the clearance between the inner turbine shell and the rotating buckets, thus affecting the operation of the turbine.
Various systems and methods are known in the art for controlling or maintaining a consistent clearance between the inner shells and rotating buckets. For example, U.S. Pat. No. 6,126,390 describes a system in which airflow from the compressor or combustor is metered to the turbine casing to heat or cool the turbine casing, depending on the temperature of the incoming air. In addition, U.S. patent publication 2009/0185898, assigned to the same assignee as the present invention, describes a system that includes an inner turbine shell having false flanges at the top and bottom to reduce eccentricities caused by transient operations. However, additional improvements in the design of casings to reduce transient eccentricities over a wide range of operating conditions would be useful.